1. Field of the Invention
This invention relates to the field of communications, and in particular to a radio receiver that is configured to receive either DSSS (Direct Sequence Spread Spectrum) or OFDM (Othogonal Frequency Division Multiplexing) communications.
2. Description of Related Art
The IEEE 802.11g specification calls for wireless communications systems that can communicate at higher data rates than conventional 802.11b devices, yet be compatible with such 802.11b devices. Within an 802.11g signal, both “serial” (802.11b-compatible DSSS) modulation and “parallel” (802.11a-compatible OFDM) modulation are employed.
IEEE 802.11b DSSS devices are configured to operate with a ‘chip rate’ of 11 Mega-chips-per-second (Mcps), and must be sampled at integer multiples of 11 MHZ, while IEEE 802.11a OFDM devices require a sampling rate of 20 Mega-cycles-per-second (MHz), or integer multiples thereof. Conventionally, dual clock-generating devices (typically crystal devices) are used to provide these sampling clock signals, and to provide the required analog-to-digital and digital-to-analog conversions of the sample streams at each sampling frequency.
PCT patent application PCT/US01/17525 “DUAL PACKET CONFIGURATION FOR WIRELESS COMMUNICATION”, teaches the use of two clock sources for dual-mode wireless communications, and is incorporated by reference herein.
The referenced patent application also proposes a scheme wherein the transmission of dual-mode communications are based on a single clock source, thereby allowing a single clock source to be used at the receivers. In proposing this single-clock based system, however, the referenced patent notes that the resultant transmitted signals are not, per se, compatible with the existing 802.11a, 802.11b, and 802.11g specification.